The present invention relates to light emitting diodes and diode lamps in which a phosphor is used to absorb and modify the primary emission from the diode. In particular, the invention relates to light emitting diodes that emit in the blue, violet, and ultraviolet (UV) portions of the electromagnetic spectrum used in conjunction with an encapsulant package that contains a phosphor that down-converts the frequencies emitted by the diode into light with a strong yellow component to produce a combined output of white light.
Light emitting diodes (LEDs) are a class of photonic semiconductor devices that convert an applied electric current into light by encouraging electron-hole recombination events in an appropriate semiconductor material. In turn, some or all of the energy released in the recombination event produces a photon.
Light emitting diodes share a number of the favorable characteristics of other semiconductor devices. These include generally robust physical characteristics, long lifetime, high reliability, and, depending upon the particular materials, low cost.
A number of terms are used herein that are common and well-understood in the industry. In such industry use, however, these terms are sometimes informally blended in their meaning. Accordingly, these terms will be used as precisely as possible herein, but in every case their meaning will be clear in context.
Accordingly, the term “diode” or “chip” typically refers to the structure that minimally includes two semiconductor portions of opposite conductivity types (p and n) along with some form of ohmic contacts through which electric current is applied to the resulting p-n junction.
The term “lamp” is used to designate a light emitting diode that is matched with an appropriate electrical contact and potentially a lens to form a discrete device that can be added to or included in electrical circuits or lighting fixtures or both.
As used herein, the term “package” typically refers to the placement of the semiconductor chip on an appropriate physical and electrical structure (sometimes as simple as a small piece of metal through which the electrical current is applied) along with a lens that provides some physical protection to the diode and can optically direct the light output. Lenses are often formed of transparent polymers and in some cases the same polymer forms an encapsulant for the diode. In the present context, the package includes a reflective structure, frequently formed of a metal or polymer within which the diode rests. Adding a lens and electrical contacts typically forms a lamp.
Appropriate references about the structure and operation of light emitting diodes and diode lamps include Sze, PHYSICS OF SEMICONDUCTOR DEVICES, 2d Edition (1981) and Schubert, LIGHT-EMITTING DIODES, Cambridge University Press (2003).
The color emitted by an LED is largely defined by the material from which it is formed. Diodes formed of gallium arsenide (GaAs) and gallium phosphide (GaP) tend to emit photons in the lower energy (red and yellow) portions of the visible spectrum. Materials such as silicon carbide (SiC) and the Group III nitrides (e.g., AlGaN, InGaN, AlInGaN) have larger bandgaps and thus can generate photons with greater energy that appear in the green, blue and violet portions of the visible spectrum as well as in the ultraviolet portions of the electromagnetic spectrum. In particular the Group III nitrides have a direct bandgap and thus generate light more efficiently than indirect bandgap semiconductors such as SiC.
In the present application, the term “white light” is used in a general sense. Those familiar with the generation of colors and of color perception by the human eye will recognize that particular blends of frequencies can be defined as “white” for precise purposes. Although some of the diodes described herein can produce such precise output, the term “white” is used somewhat more broadly herein and includes light that different individuals or detectors would perceive as having a slight tint toward, for example, yellow or blue.
As the availability of blue-emitting LEDs has greatly increased, the use of yellow-emitting phosphors that down-convert the blue photons has likewise increased. Specifically, the combination of the blue light emitted by the diode and the yellow light emitted by the phosphor can create white light. In turn, the availability of white light from solid-state sources provides the capability to incorporate them in a number of applications, particularly including illumination and as lighting (frequently backlighting) for color displays. In such devices (e.g., flat computer screens, personal digital assistants, and cell phones), the blue LED and yellow phosphor produce white light which is then distributed in some fashion to illuminate the color pixels. Such color pixels are often formed by a combination of liquid crystal elements, color filters and polarizers, and the entire unit including the backlighting is generally referred to as a liquid crystal display. (“LCD”).
Typical phosphors include minerals such as cerium-doped YAG (yttrium-aluminum-garnet). Because such phosphors are typically manufactured in the form of small particles, they must be physically dispersed as small particles on or near the diode chip. Similarly, because the encapsulant is typically a polymer resin, it typically takes the initial form of a liquid that at some point must be cast or molded into the desired shape (e.g., for a lens) and then cured into a solid form.
Accordingly, several basic combinations exist for positioning the phosphor with respect to the chip. First, the phosphor can be spread onto the chip after which the encapsulant can be added as a liquid and then allowed to cure. Although this is conceptually attractive, adding the mineral phosphor to precisely cover the chip before ever adding the encapsulant is a difficult process. As a result, its relative complexity can lower the overall rate of production while increasing the overall cost.
Theoretically, the chip could be encapsulated and then a resin coating added to the exterior of the resin, but in many cases this would produce an undesired optical result and would also prevent the encapsulant from protecting the phosphor.
Many conventional techniques for incorporating the phosphor mix the phosphor with the resin and then apply the resin-phosphor mixture to the chip. The resin is then allowed to cure with the phosphor dispersed within it.
This technique presents several challenges. First, the amount of phosphor controls the color point between the chip (e.g., blue-emitting) and the fully saturated color (e.g., yellow) of the phosphor. For the blue-chip, yellow-phosphor combination, the balance required to produce a consistent hue of white is achieved by controlling the phosphor used in the encapsulant and the amount of phosphor and encapsulant (resin) dispensed on, over, or around the chip.
As a more challenging issue, the position of the phosphor in the encapsulant with respect to the diode chip will usually affect the brightness or color uniformity of the lamp's external output. An undesired position of the phosphor can produce an undesired pattern of external output in which the output varies spatially among various shades of white.
In general, positioning the phosphor as close as possible to the chip produces the most desirable output.
Because of these and other factors, the resin-encapsulant-phosphor mixture must contain an appropriate amount of phosphor particles which are themselves of an appropriate size and in appropriate geometric relationship to the chip and to the cured encapsulant. Because the uncured resin is a liquid, however, its viscosity will affect the manner in which the phosphor will mix. If the viscosity of the resin is too low, the phosphor particles may settle within the encapsulant before it is dispensed into the package which causes undesired variation in the resulting color output. Alternatively, if the viscosity of the resin is too high, the phosphor particles will remain suspended within the encapsulant and fail to settle near the chip.
In order to deal with these difficulties, most conventional techniques attempt to maintain the viscosity of the uncured resin within a range that permits the phosphor to settle within the encapsulant under the influence of gravity. This in turn requires controlling the length of time (“working time”) during which the resin will cure—the phosphor should reach the desired position(s) before the resin cures—as well as the temperature in an effort to maintain a favorable viscosity while the phosphor is settling. For example, at room temperature (25.degree. C.), a typical 2-part silicon resin (e.g. SR-7010 from Dow Corning) will normally cure in about one minute at 150.degree. C. and its viscosity will double (from respective starting points of about 20000 and 7000 millipascal for the parts) in about three (3) hours at room temperature.
As illustrative extremes, if the size of the resin particles is extremely fine and the viscosity of the resin is quite high, the phosphor will tend to remain suspended without settling or depositing in the desired manner. Alternatively, if the particles are too large and the resin viscosity too low, the phosphor will simply sink to the bottom of the resin before it can be dispensed into the lamp package. Based on these and other factors, the choice of resin, phosphor and other variables often represents a compromise.
Accordingly, the difficulties and complexities presented by phosphor-resin mixtures create and present corresponding difficulties in the efficiency and cost of diode packaging techniques.